linux-kernel - Re: [PATCH] fs/cifs: send SMB_COM_FLUSH in cifs

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Date:	Sun, 22 Feb 2009 21:50:48 +0100 (CET)
From:	Horst Reiterer <horst.reiterer@...il.com>
To:	Steve French <smfrench@...il.com>
cc:	linux-kernel@...r.kernel.org, linux-cifs-client@...ts.samba.org
Subject: Re: [PATCH] fs/cifs: send SMB_COM_FLUSH in cifs_fsync

Hi,

On Fri, 20 Feb 2009, Steve French wrote:
> I modified your patch slightly to not lose the writeback rc in one
> case, and to change camel case pTcon to tcon and remove one
> unnecessary local variable.

Thanks.

> If you have any performance numbers before and after (with e.g. dbench,
> iozone, bonnie etc. or perhaps something which calls fsync more often
> - that would be helpful in determining whether we need a mount option
> to optionally disable it - as the samba server does)

I tested the patch using a simple test driver that transfers data in 1MB
chunks and calls fsync after each chunk. The results are as follows:

  Test run            Throughput (MB/s)
                      1 client (MB/s) 10 clients 20 clients 40 clients
  cifs - vanilla                   64        116        117        111
  cifs - fsync.patch               54         90         95         95

As you can see, fsync does make a noticeable difference here. The problem
with benchmarks in this respect is that the difference solely depends on
the underlying device. In this scenario, the storage device featured a
battery-backed disk cache. Without it, the overhead would have been much
more significant.

In any case, it's probably a good idea to introduce a mount option. The
new behavior should be enabled by default though, to provide maximum data
consistency. While I can't think of a legitimate reason to disable fsync
if data consistency is a concern, an option might be useful in the
exceptional case where 1) data consistency is not a strict requirement and
2) SMB_COM_FLUSH cannot be ignored on the server side.

Thanks,

Horst.
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